Have liver sliced 1-inch thick and cut into 2-inch squares. Clean and remove stems from mushrooms. Alternate a piece of liver, a mushroom cap, and slice of onion on 8 skewers until all ingredients are used. Mix oil, vinegar and mint thoroughly brush liver and mushrooms on both sides. Place on broiler rack about 5 inches below flame. Broil three minutes on each side or until light brown. Salt and pepper to taste. Brush with remaining liquid; broil 1 minute longer and remove from skewers. If preferred, may be served on skewers. Serves 4.